The effects of thiamine deficiency will be studied on the synthesis of acetylcholine and on the levels of cyclic AMP and cyclic GMP in rat brain. Acetylcholine synthesis will be studied by a new double label technique, using GLC-MS to measure the levels of acetylcholine. Cyclic nucleotides will be measured by radioimmunoassay. Rats will be made thiamine-deficient by feeding a diet deficient in thiamine and by treatment with the thiamine antagonists pyrithiamine and oxythiamine. They will be studied when they begin to lose weight and when they develop neurological abnormalities, and they will be sacrificed by focused microwave irradiation of the head. The degree of thiamine deficiency will be monitored by measuring the "TPP-effect" on blood transketolase. Controls will include "reversed-deficiency" (i.e. thiamine-treated) animals as well as pair-fed and normal-fed controls. Morphological abnormalities associated with thiamine deficiency will also be monitored.